I've just giffgaffed my T-Mobile USB Stick 120 (a badge-engineered ZTE MF626). I'm posting this message while using it on giffgaff PAYG, so it's costing me 3MBytes and 20p, but I think it's worth it for the good of the community.
Here's how I did it.
1. Download the unlocked firmware from here:
2. Remove your T-Mobile SIM card from the T-Mobile USB Stick 120.
(I found this slightly tricky but patience and perseverance prevailed.)
3. Put the USB stick into your PC's USB port.
4. Run the firmware updater.
It takes a little while, in my case 20 minutes, and you might worry that it's hung (I did several times) but it gets there eventually.
If you've got the T-Mobile drivers installed on your PC, these might pop up from time to time. Just close them when they appear.
The firmware updater says to use Win2k or XP. I'm running XP and it worked fine. Use with Vista or Win7 at your own risk.
5. Remove the USB stick. Insert your giffgaff SIM into it.
6. Put the USB stick back into your PC.
7. If you've got the the T-Mobile drivers installed, they will pop up and complain that they can't find an APN. Click on the "settings" button (it looks like a cog, or possibly a flower). Click "Add" to create a new configuration file.
8. Settings are as follows:
Config name: giffgaff (or whatever you fancy)
Dial number: *99# (all the pre-loaded T-Mobile settings say this here so I've copied them)
User name: giffgaff
9. Save the settings and choose "apply".
10. Click on the "Connect" button and you're away.
If you're not using the T-Mobile drivers, there's a set for 3 Australia that are meant to work well but I've not tried them. Download them from here if you need them:
Hope this helps some of my fellow giffgaffers get their mobile internet dongles working 😀